Glass is a material capable of exhibiting excellent properties such as high surface smoothness and high surface hardness. Therefore, glass is suitable for use as substrates for magnetic recording media on which information is recorded using magnetic recording devices such as hard disk drives (HDD). When a glass substrate is used as a substrate for a magnetic recording medium, it is desirable to subject the glass substrate to chemical strengthening to increase its strength.
Chemical strengthening is a technique of replacing alkali metal ions contained in the glass surface by monovalent cations having a larger ionic radius so as to form a compressive stress layer in the glass surface. Chemical strengthening is performed, for example, by replacing lithium ions (Li+) by sodium ions (Na+) or by replacing sodium ions by potassium ions (K+).
When a Pt—Fe magnetic material, which is more suitable for high-density recording than a conventional Co—Pt—Cr magnetic material, is used as a magnetic material to be deposited on a glass substrate to form a film thereon, the deposition needs to be performed at a higher temperature. Therefore, glass substrates for use as substrates for magnetic recording media are required to have superior heat resistance. Since the deposition temperature of this type of (Pt—Fe) magnetic material is 640° C. or above in the current state of the art, the glass substrates must be resistant to deformation in this temperature range.
For efficient mass production of glass substrates, it is desirable that their glass composition have properties suitable for mass production facilities for the float process or the like. Specifically, it is desirable that in the glass composition, the working temperature (a temperature at which the glass has a viscosity of 104 dPa·s, hereinafter referred to as “T4”), the liquidus temperature TL, and the difference (T4-TL) obtained by subtracting the liquidus temperature from the working temperature T4 satisfy the conditions suitable for production by the float process. The melting temperature is a temperature at which the glass has viscosity of 102 dPa·s, and is hereinafter referred to as “T2”.
Patent Literature 1 discloses a glass composition being suitable for production using mass production facilities for the float process or the like, having high heat resistance, and being suitable for chemical strengthening. The glass composition having the above-mentioned properties is obtained, in particular, by adjusting the contents of divalent metal oxides MgO, CaO, SrO and BaO, which are collectively referred to as “RO”, in terms of the effects of each of these oxides on the properties of the glass composition.
Patent Literature 2 discloses a substrate glass for data storage media having a glass transition temperature Tg of 680° C. or more and having excellent weather resistance without being subjected to chemical strengthening treatment. Specifically, the glass transition temperature of the substrate glass for data storage media is increased by adjusting the content of TiO2 and the content of ZrO2.
Patent Literature 3, Patent Literature 4, and Patent Literature 5 each disclose a substrate glass used as an information recording medium substrate or a display substrate having excellent weather resistance without being subjected to chemical strengthening treatment.